As is generally well known, slack adjuster assemblies have been employed to automatically adjust the slack in the brake rigging of a railway vehicle. A particular slack adjuster of a double-acting compression type has been manufactured by the assignee of the present invention and employed in a wide variety of railway applications. Such slack adjuster includes an elongated hollow housing member restrained against rotation and pivotally connectable at a first end thereof to such brake rigging. An elongated rod member, which is restrained against rotation and pivotally connectable at a first end thereof to such brake rigging, is also provided and has at least a threaded portion adjacent a second end thereof. The threaded portion of the rod member extends into and is reciprocally movable within the housing through a second end of the housing. A positioning means, such as a cone lock nut, movable between respective first abutting engagement, disengagement, and second abutting engagement positions is threadably engaged with the threaded portion of the rod member intermediate said ends thereof. The cone lock nut rotates about the threaded portion of the rod member when in the disengaged position thereby changing the length of the slack adjuster assembly by changing the relative longitudinal position between the housing and the rod member. A pair of opposite facing abutment surfaces positioned within a portion of the housing and a pair of opposed abutting surfaces carried by the cone lock nut for engaging respective adjacent abutment surfaces to resist rotation of the cone lock nut about the rod member when the cone lock nut is in one of the respective abutting engagement positions thereby retarding the change in relative longitudinal position between the housing and the rod member are provided. A first urging means is positioned between the cone lock nut and an abutment surface carried by the rod member for urging the cone lock nut into the disengaged position. A second urging means is positioned between the first end of the housing and the cone lock nut for overcoming a force applied by the first urging means and for urging the cone lock nut into the first abutting relationship position. An overtravel control means is positioned to reciprocally move in a longitudinal direction adjacent the first end of the housing and is engageable with the second urging means for reducing the force applied by the second urging means to less than the force applied by the first urging means, thereby allowing the cone lock nut to move to the disengaged position. An overtravel control actuating means is positioned to engage the overtravel control means for actuating the overtravel control means in response to the travel distance of a brake cylinder piston connected to such brake rigging. When the slack adjuster is triggered, the cone lock nut rotates with the sufficient speed and force that the abutting surface carried thereon frictionally engages against the abutment surface of the housing in second abutting relationship position. Such abutment surface is oriented toward the first end of the housing. In normal operation of the slack adjuster when this condition occurs, the second urging means is compressed slightly by the by the higher overtravel spring force and the cone lock nut moves from the second abutting relationship position. However, it has been found that when the slack adjuster is triggered and the elongated rod is allowed to fully extend due to improper adjustment during installation, the speed and inertia of the rotating cone lock nut is sufficient to exert greater than desirable force during contact with the abutment surface of the housing in the second abutting relationship position. Such greater than desirable contact force causes jamming condition, due to friction and, more importantly, causes operation of the slack adjuster to cease. Prior to the present invention, the jamming condition can be only corrected by manually rotating the portion of the housing disposed adjacent the first end thereof.
Therefore, there is a need for an improved slack adjuster which includes an overtravel control mechanism preventing irreversible lock-up of the slack adjuster being incorrectly installed and adjusted.